1. Field of the Invention
This invention relates to a motor controller and more particularly to such a controller having a servo system used for moving one element over a floating second element, such as a lithography stepper over a base isolated from ground motion, so as to bring the two elements to relative rest within a given time using a minimum of energy.
2. Description of The Prior Art
In the past, lithography machines have been built utilizing a large base, which base included an extremely heavy granite block firmly attached to the ground, and which block had one very flat and smooth surface. Most lithography systems typically include a mechanism, known as a stepper, for holding and moving a wafer in discrete steps over the flat surface of the granite base so that one section at a time of the wafer is exposed to energy. In the past, the energy of choice was ultraviolet light. However recent advances in semiconductor technology have resulted in the necessity to utilize shorter wavelength energy sources, such as X-rays, in order to pattern smaller and smaller features on the wafers.
The ability to pattern smaller features on semiconductors wafers has created a whole new set of constraints on lithographic systems. One such constraint is that it now is necessary to isolate the granite block from simple and everyday ground motion because of the high degree of precision required in the semiconductor fabrication processes. Such motion, for example, may result from vehicles, such as large trucks or trains, in the neighborhood where the lithographic machine is being used. With traditional lithographic machine construction, any slight ground movement is translated directly to the machine and could cause misalignments or other problems in the modern day precision machines. Further, in the case of a point source X-ray lithography system, the gap between the mask and wafer is critical and is maintained nominally at twenty microns. Thus, slight ground movements can cause problems in maintaining the integrity of the mask to wafer gap.
One solution to the ground motion problem discussed above is to isolate the granite base from the ground on isolation legs, similar in construction to shock absorbers used on heavy vehicles. Inclusion of these shock absorbers unfortunately cause a new problem of a damped oscillation of the granite base whenever the stepper is moved. Servo-controllers have been used in the past to solve these problems and generally operate by providing reverse acceleration signals to the item being moved to reduce the base oscillations. However, these techniques require significant time and energy in order to bring the entire system to rest so that it can be useful. What is needed is a more acceptable system which permits more frequent movements to occur and with the utilization of less energy.